[HN Gopher] Venus is not Earth's closest neighbor (2019) [pdf]
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Venus is not Earth's closest neighbor (2019) [pdf]
Author : BerislavLopac
Score : 226 points
Date : 2023-05-24 09:50 UTC (13 hours ago)
(HTM) web link (fermatslibrary.com)
(TXT) w3m dump (fermatslibrary.com)
| ecocentrik wrote:
| Is the point of this paper that astronomers should be more
| careful with their descriptive terminology and avoid using words
| like neighbor and neighborhood that describe static relationship
| when those relationships don't apply? If so, that's probably a
| lesson we can apply more universally.
| ttfkam wrote:
| I find it amusing seeing so many otherwise intelligent people so
| upset about a fact that contradicts their intuition. It's like
| when you tell people that if you throw a baseball from the
| International Space Station toward the Earth, the ball will end
| up in front of you and behind you at various points in the orbit.
|
| Reminds me of the quote: The universe is under no obligation to
| make sense to you.
| macspoofing wrote:
| >I find it amusing seeing so many otherwise intelligent people
| so upset about a fact that contradicts their intuition.
|
| It's not quite that. It's a semantic issue. Colloquially,
| people think of "the closest planet" as a planet having an
| orbit closest to that of earth's, whereas the article proposes
| a different definition of 'average distance' between the
| planetary bodies. Both definitions are correct, but I argue
| that the former definition is actually better as it conveys
| more information about the structure of the solar system, and
| is more intuitive.
| ttfkam wrote:
| As I said, amusing.
| George83728 wrote:
| Since you invoked the "universe making sense", I'll explain
| this using an example created by man:
|
| _" The Space Shuttle Orbiter reentered the atmosphere at
| approximately Mach 25, making it the fastest aircraft ever
| flown."_
|
| The Mach 25 reentry is an empirically verifiable fact. But
| reasonable people can object to the above assertion on
| grounds that have nothing to do with that verifiable fact;
| what exactly constitutes an aircraft and what is the proper
| way to compare the speeds of aircraft? A reasonable person
| can assert that speed comparisons between aircraft must be
| made in level flight, making the shuttle orbiter ineligible
| for such a record. Some may also object to the
| characterization of the shuttle orbiter as an aircraft, or
| the implicit exclusion of other reentry vehicles which were
| even faster. The universe does not have any objectively
| correct definition of aircraft, or any objectively correct
| way to compare aircraft speeds. These are wholly
| subjective. It has nothing to do with people _" wanting the
| universe to make sense."_
|
| Similarly, the universe does not hand us any objectively
| correct meaning of a planet's "neighbor". Nobody in this
| thread has disputed any objective empirical facts. The
| disagreement is solely around the subjective meaning of the
| ill defined term "neighbor". The disagreement is over pure
| semantics, not science.
| ryandrake wrote:
| > I find it amusing seeing so many otherwise intelligent people
| so upset about a fact that contradicts their intuition.
|
| Find a smart person who's never heard of the Monty Hall
| Problem[1] and tell them the answer without an explanation.
| They'll want to fist fight you over it. Even after you prove it
| with math, some won't believe you.
|
| 1: https://en.wikipedia.org/wiki/Monty_Hall_problem
| belter wrote:
| Technically, Earth also does not rotate around the Sun, instead
| around the center of mass of the Solar system...
|
| https://spaceplace.nasa.gov/barycenter/en/
|
| https://space.stackexchange.com/questions/9365/do-the-planet...
| ta1243 wrote:
| The barycentre of the solar system moves, sometimes it's well
| inside the Sun, sometimes it's just outside. I wonder what the
| average position is...
|
| However the Earth-Sun barycentre is always inside the Sun.
| [deleted]
| deafpolygon wrote:
| I guess, Mercury's relationship with the other planets is
| quite... mercurial.
|
| I'll see myself out.
| friend_and_foe wrote:
| If you look at the chart at the end of the article, Mercury is
| _every_ planet 's nearest neighbor by their methodology.
|
| All they're finding is "on average each planet is closer to the
| sun than any other planet" which isn't saying anything.
| anton-107 wrote:
| and is there no such pair of planets that revolve around the
| Sun while staying on the same "side" of the Sun?
| dabluecaboose wrote:
| The way orbits work, orbital velocity is tied to distance. So
| unless the planets were in the _same_ orbit, one is going to
| be going faster than the other (the closer one in a radial
| velocity sense is moving faster, and the further one in a
| tangential velocity)
| magneticnorth wrote:
| I appreciated having this pointed out to me - it's fairly clear
| once the point is made, but I'd never really thought about it.
| Of course most of the time other planets are over on the other
| side of the sun and very far away, but it's easy to just
| picture the school-style orbital map where everything is in a
| line together and closest approach = closest, period.
|
| I dunno, I thought it was neat! A slight paradigm shift in how
| I think about the solar system.
| RheingoldRiver wrote:
| SAME! I was waiting for this comment. To me this is result is
| _wonderful_! It 's like, the physical world behaving in an
| intuitive, mathematical way once you realize it. I loved
| reading this.
|
| I love it.
| ilyt wrote:
| That's like saying "Jeff isn't my closest neighbour because he
| goes to work every morning, while John WFH, so on average John is
| closer"
|
| Technically correct but not exactly first thing you think about.
| scrapheap wrote:
| So on average Mercury is closer than Venus, but if you're willing
| to wait then Venus can be closer that Mercury :)
| yieldcrv wrote:
| interesting how this describes the regional banking crisis
| jtbayly wrote:
| Of _course_ Mercury is the closest to everybody else on average.
| That 's why they made him the messenger God!
| curiousObject wrote:
| _Venus is not Earth's closest neighbor_
|
| Would it be correct to say that, on average, that statement is
| false for more than half of each Earth year?
| manojlds wrote:
| It's more like 38% of time. Mars is also in picture and it's
| about 18% of time. Mercury is about 44% of time.
| TheSpiceIsLife wrote:
| No
| beeforpork wrote:
| Kinda weird argument. The average actual distance of two planets
| on some orbits can be expected to be roughly equal to the
| distance of the farther planet from the sun (because the max.
| distance is r2+r1 and the min. distance is r2-r1, so this type of
| 'average distance' is ((r2+r1)+(r2-r1))/2 = r2), of course with
| some variation because the orbits are not truely uniform nor
| truely random.
|
| The usual notion of comparing the orbit radii is way more
| intuitive, I think.
| cratermoon wrote:
| > The PCM treats the orbits of two objects as circular,
| concentric, and coplanar.
|
| Keeping the coplanar assumption, in common with previous
| estimates, may be a problem. Mercury happens to have an orbital
| inclination of 7deg, second only to Pluto's at 1x7deg. Mercury
| _also_ orbits in an ellipse with an eccentricity of 0.21. Again,
| this is second only to Pluto, with an orbital eccentricity of
| 0.25.
|
| I'd like to see a model that takes into account inclination and
| eccentricity. For the _most_ accurate model, the concentric
| assumption also fails. Strictly speaking, none of the planets
| orbit the sun. Rather, the sun and the planet both orbit the
| barycenter of the sun-planet system, and that will be different
| for each planet. While the difference is negligible for most
| planets, for Jupiter the barycenter is quite a distance from the
| center of the sun: at 1.07 times the diameter of the sun. That 's
| just shy of 100,000km _above_ the surface of the sun.
| BlackLotus89 wrote:
| Awesome video of CGP Grey
| https://www.youtube.com/watch?v=SumDHcnCRuU talking about
| relative positions of planets
|
| Oh and don't forget the followup
| https://www.youtube.com/watch?v=LIS0IFmbZaI :)
|
| Edit just saw that someone already commented this ^^
| lc9er wrote:
| "...the closest, the mostest" is the line I always remember
| from that video.
| aaron695 wrote:
| [dead]
| passwordoops wrote:
| Reminds me of this classic SMBC about the fastest animal
|
| https://www.smbc-comics.com/?id=3457
| [deleted]
| fisian wrote:
| It just depends how you measure "closest".
|
| Similarly, Mount Everest is the highest mountain (above MSL),
| however Mauna Kea is higher if measured by prominance (starting
| from its base which is under water).
| ta1243 wrote:
| When saying Mauna Kea starts at the base, why not claim that
| with Everest?
|
| Of course in reality neither are the highest mountain, that
| honour goes to Chimborazo, a good 2.1km higher than Everest
| (when measured from the Earth's centre)
| bee_rider wrote:
| I'm pretty sure the tallest mountain in the solar system is
| Olympus Mons. Unless you measure it from the Earth's center,
| in which case I guess it would be something, anything, on
| Pluto.
| thrdbndndn wrote:
| But under what definition of "closet" would it be Venus?
|
| According to CGP Grey's video linked in the comments, not only
| Mercury has lowest average distance to Earth, it's also spend
| the most time being the closest planet to Earth, so it already
| met two definitions I can think of.
| scrollaway wrote:
| Which orbit is the closest to earths orbit.
| jjk166 wrote:
| Venus gets closer to Earth than any other planet, which is a
| perfectly cromulent definition of closest.
| jiri wrote:
| You can use delta-v, so the closest is the distance rocket
| use the least fuel/mass. It is a very good measure how easily
| to get to that destination in space.
| hpaavola wrote:
| CPG Grey, Which Planet is the Closest?:
| https://www.youtube.com/watch?v=SumDHcnCRuU
| 8note wrote:
| "Closest the mostest" makes it feel like less of a gotcha
| Blammar wrote:
| The most surprising conclusion is that Mercury is on the
| average closest to ANYTHING that orbits the sun, including
| (presumably) long-orbit comets!
| orlp wrote:
| This isn't true for two different objects in the same orbit
| with a small phase difference.
|
| To be fair that arrangement is rather unstable, especially
| with planet-sized objects.
| mr_toad wrote:
| The whirly dirly corollary.
|
| talk about cosmic apotheosis.
| yreg wrote:
| It's totally unintuitive, but makes perfect sense once you
| know the answer.
| m348e912 wrote:
| Doesn't this also imply that the sun is mostest closest to any
| planet than any other planet?
| ithkuil wrote:
| I had the same thought: if there was another planet inside
| mercury's orbit, that would be the mostest closest planet to
| all the planets, stealing mercury's status, and so you keep
| iterating on that until you reach smallest and smallest
| orbits around the sun center of mass (which is inside the
| sun).
|
| So, if you when you say "the sun" you mean the sun surface,
| then yes, the sun is always the mostest closest celestial
| body, to all planets
| manojlds wrote:
| Why do you complicate it talking of surface vs center of
| sun. Sun is sun. That's it.
| ithkuil wrote:
| just because it made it easier for me to reason about the
| fact that since the sun has some actual width it's
| exactly equivalent to a body that would orbit at that
| distance. If you frame it that way then it's rather
| obvious that the sun also fits the bill as the "mostest
| closest body" (albeit not planet)
| hgsgm wrote:
| The center of the sun has the same property, with simpler
| 0 size orbit.
| ithkuil wrote:
| it's not intuitively clear to me whether that is on
| average closer to the earth or if it's on average exactly
| as far as something orbiting around that center.
|
| Furthermore, since the sun is also orbiting around the
| shared center of mass of the whole solar system, this
| displacement albeit very small, is still enough for me to
| not intuitively understand if it makes the sun's center
| of mass closer or farther away on average than the
| closest orbiting body to the sun
| tromp wrote:
| Yes, a hypothetical planet located at the centre of the sun
| would be every planet's closest neighbour, by virtue of never
| getting as far away as others.
| ArnoVW wrote:
| Nit: located at the gravitational centre of the solar
| system. Which is not the perfect center of the sun (though
| still inside it) since all the planets pull on the sun too.
|
| Since that gravitational center, and the center of the
| pairwise systems is not the same, I wonder if a planet at
| that place is really the best solution.
| sjcsjc wrote:
| The concept of a hypothetical planet at the centre of the
| sun made me laugh. Cheers
| hibbelig wrote:
| I don't understand why we need to introduce this planet.
| Is the Sun not our neighbor? And it's closer than
| Mercury, isn't it?
| hgsgm wrote:
| Other posters seem to be treating planets as point masses
| but not the sun.
| input_sh wrote:
| > Is the Sun not our neighbor? And it's closer than
| Mercury, isn't it?
|
| It's not half the time and it is half the time. If you
| include the Sun as well as one of the possible answers
| (which I'd argue you shouldn't because neighbour implies
| same significance, not higher), the answer would've been
| an even split between Mercury and the Sun (on a large
| enough time scale).
|
| If Mercury's year somehow lasted longer than a year on
| another planet, only then would Sun be the clear winner.
| SamBam wrote:
| Actually the sun still edges out Mercury.
|
| Think of it this way: If we take the Earth as stationary
| and just look at the respective motions of the Sun and
| Mercury, then the Sun is also (roughly) stationary* and
| Mercury moves around and around it, sometimes close to us
| and sometimes far.
|
| Now, if Mercury actually yo-yo'ed through the Sun, then
| you'd be right: exactly half the time it would be closer
| to us, and half the time it would be further from us.
|
| But it doesn't yo-yo through the Sun, it moves in a
| circle. When it's 90o from us and the Sun, it's still
| further away from us than the Sun is. So it has to get
| even closer before it's equidistant. So it's actually
| closer to us only less than half of the time.
|
| *Yes, the Sun would also appear to orbit around the
| gravitational center of mass, but this doesn't affect the
| thinking above.
| Symmetry wrote:
| Which planet happens to be the closest to us right now is the
| metric most congruent to people's notion of closest and is often
| important for things like time delays in radio communication. By
| that metric it's currently Mercury, closely followed by Venus[1].
| But it will change and keep changing.
|
| In terms of how hard it is to get places you really want a delt-v
| map[2]. By that metric Venus is the closest at 640 m/s from Earth
| intercept to Venus intercept.
|
| It's sort of interesting that, over an indefinite period of time,
| Mercury is closest on average but that doesn't really correspond
| to our intuitive notion of "closest" nor is it a particularly
| useful metric for anything that comes to mind. So the whole
| gotcha here is really pretty silly.
|
| [1]https://www.theplanetstoday.com/
|
| [2]https://i.imgur.com/AAGJvD1.png
| MayeulC wrote:
| > By that metric Venus is the closest at 640 m/s from Earth
| intercept to Venus intercept
|
| By the way, I think there is a typo on that delta-v map. I
| doubt low Venus orbit to Venus is 27km/s, vs 9.4 for the earth,
| when Venus gravity is just 90% that of Earth.
| mnw21cam wrote:
| Depends on whether you take atmospheric drag into account. If
| you do, then you'll be fighting Venus' thick atmosphere all
| the way up, and that 27km/s figure could well be accurate.
|
| I don't like it when delta-v maps include atmospheric drag,
| because the numbers depend on how aerodynamic your rocket is,
| in contrast to the other manoeuvres where the amount of
| delta-v doesn't depend on the type of rocket you have at all.
| vikingerik wrote:
| As other replies are saying, that does include the
| atmospheric drag. But that number wouldn't actually happen
| with rocket thrust. You'd never actually do that, launch a
| rocket from the surface of Venus - you'd first take
| advantage of the atmosphere to do aerodynamic flight, first
| carry the rocket to much higher altitude with an airplane
| and then ignite it from there.
| ooboe wrote:
| Didn't work out so well for Branson.
| mlyle wrote:
| Worked just fine for Orbital/ATK Pegasus.
| mnw21cam wrote:
| It would be much more of an advantage on Venus than it is
| on Earth.
| TeMPOraL wrote:
| Here is the OG source of those numbers: https://old.reddit.
| com/r/space/comments/1ktjfi/deltav_map_of....
|
| Skimming the thread, they made "assumptions" for cases like
| taking off from bodies with atmosphere.
|
| Here is more on how they came up with the number for Venus,
| including some actual math: https://old.reddit.com/r/space/
| comments/1ktjfi/deltav_map_of....
|
| The OG image mentions that there are assumptions being
| made. The image linked by GP is a derivative work,
| improving on and crediting the work of /u/CuriousMetaphor,
| however it omits some of the caveats in the legend.
| MayeulC wrote:
| Thank you for pointing this out.
|
| > I don't like it when delta-v maps include atmospheric
| drag
|
| Yes, I find it quite unintuitive, especially as the map is
| now asymmetric: if you take into account drag on liftoff,
| you would also take into account aerobraking for reentry.
| It means that the map can't really be used for body-to-body
| calculations, as it assumes "rocket liftoff" for the low
| orbit<->surface transition.
|
| Ideally, atmospheric parameters should be specified some
| other way on the map, or it could branch to show both
| liftoff and reentry costs on each body (and possibly
| delta-v due only to gravity).
| mlyle wrote:
| It's just the first segment on the trip.
|
| Reentry delta-V isn't really well-posed. The delta V that
| would enter orbit, or even less, with a somewhat
| different angle reenters. So the "reentry delta V" might
| very possibly be negative, in that you could go Earth LEO
| to body surface with less velocity change.
|
| > possibly delta-v due only to gravity)
|
| Now, that's more useful to have around.
| Symmetry wrote:
| The numbers for reaching LEO include typical atmospheric drag
| losses. That's only a km/s or so on Earth but on Venus with
| it's very thick atmosphere the losses are much higher.
| mr_toad wrote:
| From the surface? It's unlikely that anything would every
| launch from the surface of Venus.
| jjk166 wrote:
| Probably more likely than a launch from the surface of
| Jupiter.
| mlyle wrote:
| It's not only unlikely, it's unpossible.
|
| If you're using chemical propulsion, you're not going to
| get much more than 20km/sec even with a whole lot of
| stages.
| Symmetry wrote:
| You'd certainly need something beyond a chemical rocket
| but there are options. At that density a turborocket[1]
| would certainly be worth the extra mass but I think still
| wouldn't be enough of an advantage. In the realm of
| roughly existing technology, a nuclear ramjet[2] could
| get you to the upper reaches of the atmosphere and give
| you a nice little initial boost as well to your speed.
| And in the realm of SF, a nuclear saltwater rocket[3]
| would still be easily capable of making it out of Venus
| in one stage.
|
| [1]https://en.wikipedia.org/wiki/Air_turborocket
|
| [2]https://en.wikipedia.org/wiki/Project_Pluto
|
| [3]https://en.wikipedia.org/wiki/Nuclear_salt-
| water_rocket
| mlyle wrote:
| > At that density a turborocket[1] would certainly be
| worth the extra mass but I think still wouldn't be enough
| of an advantage.
|
| How's a turborocket work without free oxygen gas in the
| atmosphere? I mean, maybe fluorine, but I doubt you come
| out ahead that way.
|
| Density effects, though, make balloon-launched rockets,
| etc, conceivable.
| Symmetry wrote:
| To get a turborocket to work in an atmosphere without
| oxygen you just have a classic rocket engine, with its
| own fuel and oxidizer, use its exhaust to drive a turbine
| the same way a jet is used to drive a turbojet. You have
| to leave off the afterburner stage that many existing
| turborockets have where you inject more fuel to burn
| after the final turbine.
| mlyle wrote:
| If you're using air for just working mass and not for an
| oxidizer, it's my belief that this does not improve
| specific impulse but only improves peak thrust/engine
| weight.
| distortionfield wrote:
| Did you read to the end? The point of the paper is not the
| gotcha about which planet is really closest. Their point is
| that their PCM method allows you to quickly estimate distances
| between groups of planetary bodies in a novel way. They aren't
| trying to be "gotcha" about it, they're introducing a new model
| for estimating solar distances.
| ryanmcbride wrote:
| This exact reasoning is why I've always been kind of annoyed by
| the song Bitch Don't Kill My Vibe by Kendrick Lamar. In the
| chorus he goes "I can feel your energy from two planets away"
| and even though I know art doesn't have to conform to
| scientific reality, poetry and music almost especially, it
| ALWAYS bugged me from the first time I heard it til today. Like
| what does 2 planets away even mean? that's a hugely variable
| distance.
| hinkley wrote:
| We're heading for Venus And still, we stand tall
| 'Cause maybe they've seen us And welcome us all, yeah
| With so many lightyears to go And things to be found
| I'm sure that we'll all miss her, so It's the
| final countdown
|
| Someone's science teacher knows who was too busy writing
| lyrics in class to learn about outer space...
| VincentEvans wrote:
| Venus is just the first stop, duh.
| olddustytrail wrote:
| Heh, reminds me of the Simon Singh criticism of Katie Melua's
| "Nine Million Bicycles".
|
| Which she followed up on
| https://m.youtube.com/watch?v=21iUUe-W8L4
| yazantapuz wrote:
| It means that is a huge distance, in human terms. Hugely
| variable, but huge at the end.
| nuncanada wrote:
| Yup silly.. And people consider the closest mostly in the sense
| of the closest distance between orbits not distance between
| planets...
| akolbe wrote:
| Basically, the average distance between one planet and another
| will always be greater than the distance between the respective
| planet and the sun. (Note that in the table on the last page,
| all average planet distances for earth are > 1 AU.) Thus
| whichever planet is closest to the sun will always be the
| closest on average to any other planet.
| Asraelite wrote:
| This is true for the Solar System because all our planets
| have near-circular orbits. Suppose though if you had two
| planets with highly elliptical orbits and similar arguments
| of perigee. They will spend most of their time far away from
| their star but relatively close to one another.
|
| I'm not sure if such an elliptical orbit would be possible
| while still classifying them as planets and not dwarf planets
| though.
| CaptainNegative wrote:
| Outside of binary planets orbiting each other, is this
| achievable as a (mostly) stable configuration?
|
| Intuitively, if the two planets have orbiting periods that
| are not basically identical, then after long enough they
| will also have long stretches of time where they are on
| opposite sides of the star (with a slight caveat if the
| periods are rational multiples of each other, but in either
| case their positions will be asymptotically uncorrelated).
| On the other hand, if they are close and have the same
| period, I'd expect their gravitational pulls would
| eventually merge them together unless they become a binary
| planet system.
|
| But I have no physics/astrophysics background so this could
| easily all be stupid.
| nickparker wrote:
| IIRC we've found some big exoplanets in wild orbits. They
| tend to _form_ in a circular-ish orbit because that 's how
| you get a big enough chunk of the dense bits of an
| accretion disk, but interactions with other planets/stars
| can put eg gas giants into wild orbits after formation.
|
| EG https://astronomynow.com/2019/08/28/exoplanet-found-in-
| unusu...
| Mystery-Machine wrote:
| Almost true. Good thinking though. There are exceptions to
| this rule, so it's more of a guidance than the actual rule.
| For example you could have two planets orbiting the Sun at
| the relatively similar distance from the Sun and a small
| distance from each other.
| CydeWeys wrote:
| You couldn't actually have two separate planets with
| similar orbital distances like that though.
| digging wrote:
| Right. They would necessarily have different orbital
| periods because the size of an orbit is directly
| proportional to the time it takes to complete one orbit:
| https://en.wikipedia.org/wiki/Orbital_period#/media/File:
| Sol...
|
| This is why objects in geostationary orbit can only exist
| at a particular distance from the earth:
| https://en.wikipedia.org/wiki/Geostationary_orbit
| zamadatix wrote:
| That's still the same scenario, just slower to become
| apparent. You need to change other things, like they orbit
| each other as they go around the Sun, to change this.
| BurningFrog wrote:
| Before morning coffee:
|
| Over an indefinite period of time, I'd expect that all planets
| are on average placed in the center of the Sun, and equally far
| away.
|
| Now:
|
| Oh, right. It's not the distance to the average, it's the
| average of the distance.
| madcaptenor wrote:
| And the error in your approximation goes up as the orbit gets
| bigger, which recovers the original result.
| dr_dshiv wrote:
| Which planetary orbit is closest to Earth's? That's what I
| think people are meaningfully thinking.
| ianai wrote:
| Venus by that measure.
| svachalek wrote:
| Yup, that's equivalent to the delta-v definition afaik.
| (Venus)
| Symmetry wrote:
| It's close equivalent. It's easier to go inwards than
| outwards in terms of delta-v because orbits further out
| move more slowly. So if things were a little different the
| orbit of Mars could have been closer to Earth in terms of
| distance while Venus would still be closer in terms of
| delta-v.
| adql wrote:
| > In terms of how hard it is to get places you really want a
| delt-v map[2]. By that metric Venus is the closest at 640 m/s
| from Earth intercept to Venus intercept.
|
| Well, that's also a minimum that changes over time. It might be
| closest at a particular moment but still not "on average"
| 2d8a875f-39a2-4 wrote:
| Very droll. Reminds me that "teaspoon half-life" paper.
| Ekaros wrote:
| Which planet on average have shortest travelable route? That is
| not one going through huge hot blocking object.
| ta1243 wrote:
| Shortest measured how?
|
| As a photon flies? Mercury.
|
| In terms of energy required to get there? Venus.
|
| In fact Mercury is the _furthest_ planet in terms of energy
| required -- it 's easier to get to Neptune than Mercury.
|
| In time to get there assuming minimum energy and no gravity
| assists from other planets? Venus I think, but maybe Mars.
| short_sells_poo wrote:
| I suppose another very loosely defined notion of closeness is
| the degree of human habitability, in which sense Mars might
| be closest. Both Venus and Mercury are incredibly hostile
| environments, although one could argue for floating cities on
| Venus and a thin habitable zone at the poles of Mercury.
|
| I understand of course that anywhere "not on Earth" is
| incredibly hostile to Human life, at least what we can see
| with present day technology. For truly habitable planets, we
| might have to consider other star systems and even then
| there's no guarantee we'll find one.
|
| I don't know to what degree is the abundance of oxygen as a
| loose element a sign of life, but I'd expect it to be bound
| to minerals anywhere without significant plant-like life.
| Perhaps finding another habitable planet is the same task as
| finding life on another planet?
| vikingerik wrote:
| And why is Mercury so hard to get there in terms of energy
| required? It's because its orbital velocity is so fast and
| you need to match that. (You can get there without matching
| the orbital velocity, but that won't be useful, you're either
| doing a non-capturable flyby or a very hard impact.)
|
| Mercury's orbital velocity is 48 km/s. Earth's is 30. An
| object at infinity would be zero. Kinetic energy is
| proportional to velocity squared. Square those numbers and
| you see the energy differential between Earth's orbit and
| Mercury's is greater than going from Earth to infinity.
| pimlottc wrote:
| TL;DR it's Mercury
|
| The article describes a novel mathematical formula for
| calculating the average distance between planets orbiting the
| same star. Using this method (and confirming with computer
| simulations), the authors determine that Mercury, not Venus, is
| the closest planet, on average, to Earth.
|
| It's actually the average closest for all the planets! However it
| does not really go into the orbital mechanics to offer any
| intuitive explanation of this surprising result.
| nordsieck wrote:
| > However it does not really go into the orbital mechanics to
| offer any intuitive explanation of this surprising result.
|
| I think you just need geometry to get an intuitive explanation.
|
| 1. Draw the Sun and the orbits of Mercury, Venus, Earth, and
| Mars.
|
| 2. Pick a point on Earth's orbit for where Earth is.
|
| 3. Draw a circle around Earth that intersects the Sun.
|
| 4. Draw a line, tangent to that circle, that intersects the
| Sun.
|
| A hypothetical planet that orbits the sun at 0 distance has
| half of its orbit in the circle and half of its orbit outside
| the circle.
|
| As planets get further from the sun, an increasing proportion
| of their orbit sits outside the circle.
| xmcqdpt2 wrote:
| The "paper" looks like a science fair project to me (a very
| good one though!) They don't really come up with a new method
| of calculating the distance, more like an explanation that
| might be more intuitive to some.
|
| The average distance is pretty straightforward to calculate
| over all times using integration.
| eimrine wrote:
| because other planets use to appear too far sometimes, as it
| has been shown by whirly-dirly simulation
| fingerlocks wrote:
| The title should include _On Average_ , but that makes the
| article less clickable. I wonder if they'll update the title when
| Venus whips by later this year.
| aexol wrote:
| Great discovery ;)
| plank wrote:
| Yes, play of words. And with a choosen weight function.
|
| If, e.g., the weight function would not be 'sum over all
| distances in a given timeframe with the same weight' but for
| instance '... with weight 1/(distance^2), the results would be
| different (mercury would not win for each planet).
|
| I guess if someone asks, 'which neighbour' is closest, I would
| say the neighbour living literally next door, even though on
| workdays our distance is much larger (as we work in different
| cities) then that other neighbour three blocks down who works in
| the same city as myself.
| NovemberWhiskey wrote:
| I suspect that if you're defining "closest neighbor" using
| anything other than a delta-v budget, you are probably doing it
| wrong.
| [deleted]
| MrQuincle wrote:
| I guess an even closer neighbor is the sun.
| bell-cot wrote:
| Yeah - casually ruling out the moon and sun in the article's
| first sentence feels a bit underhanded. Beyond pedantry, what's
| the point of the "Planets Only" sign?
| dmurray wrote:
| Not as close as the moon.
| billpg wrote:
| But not as high as maybe dirigibles or zeppelins or
| lightbulbs. And maybe clouds.
|
| I'm sorry. I'm not sure what happened to me there.
| jasonlotito wrote:
| The difference between closest planet on average versus closest
| orbit.
| mikeInAlaska wrote:
| Mercury by average. I've wondered in the past it we could land
| people on Mercury... at the right distance from the sunset
| terminator where the surroundings had cooled to, say 21 degrees
| C, (70F). With a 1408 (Earth) hour day, the landing party could
| stay for quite a while before needing to relocate again towards
| the terminator, or leave.
| given wrote:
| [flagged]
| throwaway5752 wrote:
| In the commonly understood way, orbitally, Venus definitely is
| Earth's closest neighbor.
|
| I appreciate pedantry as much as the next person, and it's an
| interesting article, but this article article and the approach
| outlined would not aid survival to an interplanetary colonist.
| They could enjoy being technically correct in the short remainder
| of their life, while being carbonized. And to follow up pedants,
| I am well aware of surface conditions on Venus.
| varenc wrote:
| It's interesting that Mercury is _every_ planet's closet
| neighbor. And of course its orbit is so close to the Sun that
| that average distance to Mercury is about the same as each
| planet's average distance from the Sun. (Which I think is even a
| closer average neighbor)
| shmerl wrote:
| Supposedly Mercury is closest to every other planet at some
| point. But since distances change all the time, there is no
| constant "closest".
| fnovd wrote:
| This is only true when it is meaningless.
|
| Let's say my friend was born on January 2, 1900, and I was born
| on January 1, 1900. Who has the closer birthday to someone born
| on January 1, 2000?
|
| If you say it's me, well guess what, you're wrong! My birthday is
| 1 day farther away, just count the days.
|
| If you say it's my friend, well guess what, you're wrong! January
| 1 == January 1. It's the same birthday, obviously.
|
| Incidentally this is why I think LLMs are a lot more impressive
| than many here give them credit for. Language is ambiguous and
| being able to create something novel & coherent using an
| algorithm is an amazing achievement. Embedded in our use of words
| is a lot of information that is often incongruent with the
| understood meaning of the word, as this example shows. Apparently
| humanity itself has two divergent "hallucinations" of what the
| word "closest" means in this context. We really can't blame the
| AI for imitating us when that's what we ask it to do.
| jalk wrote:
| Oh, this perhaps explains why my horoscope is wrong a lot of the
| time
| pmontra wrote:
| Of course it's Mercury but even after I explain it to people and
| they understand the point, they keep saying that Venus is
| closest. Probably people are used to think about distance between
| stationary places so they look at the orbits, not to where
| planets are on their orbit.
|
| By the way, that probably explains a lot of sci-fi movies where
| they have to go to Mars first, then Jupiter, then Saturn, then
| Neptune.
| darkwater wrote:
| I think that the trick - that it's not automatic for most
| people - is "on average". I guess people tend to think about
| the minimum distance rather than the average.
| adwn wrote:
| > _Probably people are used to think about distance between
| stationary places so they look at the orbits, not to where
| planets are on their orbit._
|
| These people are more correct than you give them credit for.
|
| 1. If you look at their orbits (and not momentary positions),
| then the orbit of Venus is closer (less distance) to the orbit
| of Earth than the orbit of Mercury is.
|
| 2. It takes less delta-v to go from Earth to Venus than to
| Mercury.
| Slow_Dog wrote:
| Venus has the smallest average distance from each point of the
| Earth's orbit to the nearest point on the other planet's orbit.
| I.e. whenever we're shown a diagram of the solar system, the
| "circle" that is Venus's orbit is closest (has the most similar
| size) to that of Earth's. I don't know the correct term for
| this measure.
| amelius wrote:
| But is Earth Venus' closest neighbor?
| yreg wrote:
| Everyone's closest neighbor is Mercury by this methodology (or
| Sun if you count it).
| quickthrower2 wrote:
| The paradox is because we use neighbour to mean something
| else. Imagine 10 RVs doing road trips through SE Asia. Most
| do extensive tours across multiple countries. One of them
| stays around a small town the whole time but is on average
| the closest to any given other van, but never that close to
| really be called a neighbour but then we call it a
| "neighbour"
| NeoTar wrote:
| This question came up when I was studying for my PhD, and had
| access to software with built in solar-centric planetary
| locations. From what I recall, across the full-range of time
| available in the software (which was something like 300 years) it
| did apply that the Mercury-[PLANET] distance was minimal,
| although possibly not for Pluto since 300 years is only like one-
| and-a-quarter orbits.
| dav_Oz wrote:
| Distance: a _straight_ line (i.e. _shortest_ line in Euclidean
| geometry) between two points - is a one dimensional object
| /projection. On Earth crust where we evolved as 1 m long objects
| immersed in 1 atm of mostly nitrogen at around 290 K and at a
| constant pull at 1 g on a spinning ball at 11.5 uHz ... you get
| the idea ... a very straightforward concept.
|
| Applying it to dust and gas particles (massive enough for gravity
| to shape it into a spinning sphere) in a vacuum falling towards
| (on a stable enough orbit) a gas ball with 99.8% of the total
| mass in the system, itself in hydrostatic equilibrium with its
| own gravity pull through nuclear fusion of hydrogen into helium
| ... Well, we are not used to that kind of _motion_ i.e. geometry
| on astronomical scales. But once you see what dictates the (near)
| stable motions of all those tiny particles the solution is
| simple: in the _solar_ system the _sun_ is nearest to all objects
| in the system, on average. Whatever is nearest to the Sun is the
| closest on average to all other elliptical moving objects. From
| Neptune 's perspective Mercury is barely moving and oscillating
| so fast as to standing still. So, in the case of planets the
| underlying geometry is moving along (stable) elliptical (i.e.
| closed path) orbits (approx. Newtonian mechanics), applying our
| "straight" lines for "distance" is amiss here and can lead to
| confusion. I personally like to see it more - inspired through
| e.g. Kepler's imagination[0] - as a geometrical-dynamic system
| (Kepler orbits) more akin to _music_.
|
| [0]https://en.m.wikipedia.org/wiki/Harmonices_Mundi
| sbussard wrote:
| This is academic click bait. No astronomer is shocked by these
| conclusions. It is a waste of time.
| hgsgm wrote:
| Not everyone is an astronomer.
|
| Not everything needs to be "shocking".
| knome wrote:
| it is interesting that earth spends more time with mercury closer
| than venus due to orbital mechanics, but the entire premise of
| the article is just an annoying "gotcha" twist of language.
|
| the planet earth is ever nearest to is Venus, which is what
| people will mean by "closest neighbor". if you work from home and
| your next door neighbor works at the office, it doesn't make the
| retired lady in the next house over your closest neighbor,
| regardless of spending more time in closer relative proximity to
| her.
| manojlds wrote:
| But in this case the house itself is moving.
|
| If Mercury and Venus had people, which of those people is our
| closest neighbour?
| otabdeveloper4 wrote:
| Whichever one has smaller delta-v.
| adql wrote:
| The one with smallest ping
| dreamcompiler wrote:
| > it doesn't make the retired lady in the next house over your
| closest neighbor, regardless of spending more time in closer
| relative proximity to her.
|
| It does matter a lot if you're trying to send a rocket to a
| particular one of those neighbors at a particular time.
| zamadatix wrote:
| In that case you probably want a delta-v not distance.
| mercuryftw123 wrote:
| I don't think that way. It's more like this: your two blocks
| down the street neighbor drives by you every morning, would you
| classify it being closer to you? Because that's how we look at
| the Venus at the moment. Mercury should be the real neighbor.
| vertis wrote:
| I think a better example is neighbour 1 drives past your
| house once a day and neighbour 2 drives past 10 times a day
| on the main road several blocks further from your house. If
| you needed a lift somewhere (or some other reason to
| interact), you can either wait patiently for N1 to drive past
| or you can walk the several blocks to the main road, and then
| wait less time for N2.
| inopinatus wrote:
| If we're talking about transportation, well then. The
| efficient transfer to Mercury is via Venus transfer, so it is
| farther by such reckoning. In terms of delta-v budget, Venus
| is actually closer than Mars, unless you wanted to, say,
| visit the surface.
|
| See https://upload.wikimedia.org/wikipedia/commons/9/93/Solar
| _sy..., essential information to plan your next
| interplanetary holiday
| aidenn0 wrote:
| So to stretch the analogy to its limits, what if you have
| someone who lives directly across the freeway from you, and
| a neighbor whose house is next to the overpass that lets
| you cross the freeway.
| 0xFF0123 wrote:
| Taking your argument to the extreme, if there was a planet that
| somehow brushed by extremely closely to Earth every thousand
| years, that planet would be the closest neighbor? I would argue
| not. I think the average is more meaningful.
| dahfizz wrote:
| The term "neighbor" here is confusing. Venus has the
| planetary orbits right next to Earth's. That makes them
| "neighbors" - they live right next door from each other.
|
| Mercury is the closest planet on average, but calling it the
| closest neighbor is just confusing.
| pleb_nz wrote:
| Agreed, that was my initial thought and I was going to
| comment the exact same thing as you and then I decided to
| check up the meaning if neighbour. Turned out I've
| misunderstood the meaning my whole life and it doesn't just
| mean next to, it means nearby.
|
| https://www.merriam-webster.com/dictionary/neighbor
|
| So I guess that means venus's orbit is our closest
| neighbour, but not the planet itself most of the time....
| kwhitefoot wrote:
| > doesn't just mean next to, it means nearby.
|
| That's why we say "next door neighbour"
| forbiddenvoid wrote:
| This is exactly my thinking. In this context, I think the
| paper depends on an implied ambiguity of the word
| 'neighbor' and how it's used in the context of orbiting
| bodies.
|
| When I think of 'neighbors' in the context of the solar
| system, I am generally thinking of neighboring orbits. It
| would be hard to argue that Venus's orbit is not the
| closest orbit to Earth's. Or at least it would seem silly
| to do so.
|
| Maybe I'm being overly pedantic here, but my view is that
| orbits have neighbors, and planets have orbits, but planets
| don't necessarily have neighbors. Something in the word
| 'neighbor' implies persistence to me, so I don't really
| consider average-closest planet to be a neighbor when
| exactly which planet is closest to any other changes
| constantly.
| thx-2718 wrote:
| Both have independently useful meaning.
|
| The average closest planet would be useful for regularly
| traveling between two places.
|
| The closest at any one time is useful for planning
| intermittent trips.
|
| Let's say we had to pick 2 equal planets we would travel to.
| Adding the stipulation that planet A is closer on average to
| HOME than planet B; Planet B has the shortest distance to
| HOME.
|
| If travel is cheap then planet A is more useful to travel to
| since you can afford to do it regularly.
|
| If travel is expensive then planet B is more useful since you
| can't afford travel all the time and rather need to make sure
| each trip is worth.
|
| If you live in a city it's more reasonable to go to the
| grocery every week.
|
| If you live in a rural area it's more reasonable to wait for
| the Shwan truck (type of grocery delivery) once a month.
| Ensorceled wrote:
| To return to the analogy, if once a year my brother-in-law
| parks their RV in my drive, nobody would be confused about
| who my closest neighbour is.
|
| I would argue that MOST people would disagree with you and
| would say Venus is our closest neighbour but once every 1000
| years wobbly Erebus returns from the dark to scare the hell
| out of us.
| tectec wrote:
| What kind of orbit would that be? In order for it to be that
| close so rarely it must spend most of its time even further
| from earth.
| adgjlsfhk1 wrote:
| Haley's comet is a good example. it gets really close every
| 87 years
| playingalong wrote:
| 75
| Someone wrote:
| 74-79, with an average of 76
|
| https://solarsystem.nasa.gov/asteroids-comets-and-
| meteors/co...:
|
| _"Halley 's orbit period is, on average, 76 Earth years.
| This corresponds to an orbital circumference around the
| Sun of about 7.6 billion miles (12.2 billion kilometers).
| The period varies from appearance to appearance because
| of the gravitational effects of the planets. Measured
| from one perihelion passage to the next, Halley's period
| has been as short as 74.42 years (1835-1910) and as long
| as 79.25 years (451-530)."_
| BurningFrog wrote:
| Halley's Comet might fit that description.
| digging wrote:
| By what metric is Halley's Comet a planet though?
| Someone wrote:
| Halley's Comet doesn't necessarily get close to earth.
| https://solarsystem.nasa.gov/asteroids-comets-and-
| meteors/co...:
|
| _"During its 1986 appearance, Halley 's nearest approach
| to Earth occurred on the outbound leg of the trip at a
| distance of 0.42 AU (39 million miles or 63 million
| kilometers)"_
|
| (The orbit of Venus is at about 0.7 AU, so 0.3 AU from that
| of earth, so that was further away than Venus _can_ be to
| earth (https://theskylive.com/how-far-is-venus))
|
| It can get a lot closer, though. From that nasa.gov page:
|
| _"The comet 's closest approach to Earth occurred in 837,
| at a distance of 0.033 AU (3.07 million miles or 4.94
| million kilometers)"_
|
| That's about 13 times the earth-moon distance.
| hgsgm wrote:
| I don't understand your language.
|
| 1986 Halley's comet was closer to Earth then Venus is
| close to Earth whenever Venus is on the other side of the
| Sun from Earth
| aidenn0 wrote:
| I had to read their sentence like 5 times to get it as
| well.
|
| .42AU is a larger distance than the closest that Earth
| and Venus ever get to each other (i.e. closest than as
| close as it is possible for Venus to be, with the word
| "can" being used in this meaning)
| xwdv wrote:
| If I have a house and you have a house next to me yet neither
| of us are ever home, we are still neighbors no matter where
| in the world we are.
|
| Similarly, the entire area earth and Venus clear with their
| orbits is the planet's "home", therefore, we only have two
| neighbors, Mars and Venus, and Venus is probably the closest.
| hgsgm wrote:
| It doesn't take me a year to walk a lap around my home.
|
| Maybe you could say that the orbit is territory that a
| planet roams, like a nomadic person or migratory bird.
| davidgrenier wrote:
| I had never heard of the concept of closest average neigbor
| before. When I read it, I assumed it meant: draw a straight
| line between the two bodies, that is the current distance
| between the two bodies, now average it over a couple of years
| of motion of these two bodies in space.
| rini17 wrote:
| As light flies yes. But not closest to get stuff there (delta-v).
| Probably that's where Venus comes in.
| perihelions wrote:
| I think you can replace the whole analysis with a single diagram.
| You don't actually need to find the closed-form solution in terms
| of elliptic integrals; the only part we use is whether the
| integrals have a certain monotonicity property (smaller radius =>
| smaller mean distance). And you can rearrange the "mean distance"
| integrals in way that the rearranged integrands are pointwise
| monotonic; and the proof that they're monotonic is an elementary
| geometric one.
| perihelions wrote:
| And here it is, proof in one picture:
|
| https://i.ibb.co/kxjDbWP/a.png
|
| Pure classical geometry (I think?)
|
| The integral over the circle can be rewritten as an integral of
| a sum of two terms over a semicircle - the term for the local
| point, plus the term for the mirrored point on the other
| semicircle. This sum-term is an everywhere-monotonic function
| of the radius of the circle (in the proof diagram: XA + XB > XC
| + XD).
|
| (XAQB, XCRD, and XC1QD1 are constructed as parallelograms.
| XC1RD1 doesn't mean anything; it's just a construction whose
| perimeter compares easily against the other two).
| jjk166 wrote:
| We speak a language made for medieval peasants to talk behind
| their lords' backs, it's hardly surprising that our vocabulary
| leads to ambiguity when talking about orbital mechanics.
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